Wind turbine lightning protection system

ABSTRACT

A wind turbine blade comprising a lightning protection system, which is at least partly disposed in an inboard portion of the blade. The lightning protection system comprises a down conductor cable having root and tip portions having insulation with different electrical breakdown voltages. The lightning protection system may comprise a down conductor cable portion and a supporting component to hold the cable portion in free space in a position near or on a camber line of the blade aerofoil section, so that the cable portion is spaced apart from at least one electrically conductive structural component; and/or a plurality of inboard down conductor cables and a diverging electrical junction.

FIELD OF THE INVENTION

The present invention is in the field of wind turbines, specificallywind turbine lightning protection systems.

BACKGROUND OF THE INVENTION

Wind turbines can be struck by lightning. The electric current within atypical negative cloud to ground lightning discharge rises very quicklyto its peak value in 1-10 microseconds, then decays more slowly over50-200 microseconds. This electric current, when striking the blade of awind turbine, travels to ground via the lowest impedance path. Withoutany lightning protection, this path generally includes electricallyconductive components of the wind turbine. A large electric currentthrough such a component has the potential to cause damage to thecomponent. In order to protect components of the wind turbine, lightningprotection systems have been developed. A wind turbine blade having onesuch lightning protection system is shown in FIG. 3 .

SUMMARY OF THE INVENTION

It has been identified that known lightning protection systems can beimproved by the following aspects of the invention, in which electricitypassing from a tip of the blade passes through the lightning protectionsystem. In the following aspects of the invention, the likelihood ofelectricity passing through other electrically conductive components ofthe blade, such as electrically conductive structural components, issignificantly reduced.

There may be an electrically conductive component of the blade, such asa spar cap comprising carbon fibres. There may be a potential differencebetween the electrically conductive component and one or more downconductors of the blade extending in proximity to the structuralcomponent. In the event of a lightning strike causing an electricalcurrent in the down conductor it may be desirable to avoid electricitypassing through the electrically conductive component. It may bedesirable to avoid a current or attachment to the electricallyconductive component as the current may not flow in a reliable orpredictable path through the component, and it can be difficult to modeland predict the effects of likely current paths through the electricallyconductive component.

A first aspect of the invention provides a wind turbine bladecomprising:

-   a root end and a tip end, a spanwise dimension extending between the    root and tip ends; and an inboard portion extending up to 50% of the    spanwise dimension from the root end towards a midpoint between the    root and tip ends;-   a lightning protection system comprising a down conductor cable at    least partly disposed inside the inboard portion of the blade,-   wherein the down conductor cable comprises a cable first portion and    a cable second portion, the cable first portion being closer to the    root end of the blade than the cable second portion, each cable    portion comprises insulation which has a respective electrical    breakdown voltage, wherein the cable first portion insulation    electrical breakdown voltage is higher than the cable second portion    insulation electrical breakdown voltage. The insulation may comprise    or consist of a dielectric material.

By providing a down conductor cable first portion having a higherinsulation electrical breakdown voltage, in the inboard portion of theblade, closer to the root end than the second portion, any interactionbetween the lightning protection system and any electrically conductivecomponents in the blade are minimised. This is particularly effectiveclose to the root end where the potential difference between the downconductor cable and any conductive component in the blade is at itshighest, in the event of a lightning strike.

Expressed another way, the insulation of the cable first portion has afirst dielectric strength value and the insulation of the cable secondportion has a second dielectric strength value. The first dielectricstrength value is higher than the second dielectric strength value.

The expression “an inboard portion extending up to 50% of the spanwisedimension from the root end towards a midpoint between the root and tipends” means that the inboard portion starts at the root end and extendsto a point on the blade no further than 50% of the spanwise dimensionfrom the root. In other words, the inboard portion is only in the firsthalf of the blade length as measured from the root. Expressed anotherway, the inboard portion has a proximal end at the blade root and adistal end, the distal end being positioned no further than 50% of thespanwise dimension from the root.

The cable first portion may comprise insulation which has an electricalbreakdown voltage at least 1.2 times higher than the electricalbreakdown voltage of the cable second portion insulation, preferably atleast 1.5 higher, and more preferably at least 1.6 times higher.

As an example, the cable first portion may be the cable PT6-CU50 HV500DCand the cable second portion may be the cable PT6-CU50 HV300DC (bothcables from PolyTech A/S, Denmark).

The difference in electrical breakdown voltage of the insulation of thecable first portion and the cable second portion may arise from one ormore of: thickness; material; or shape.

The spanwise dimension may have an outboard portion extending up to 50%of the spanwise dimension from the tip end to a midpoint between theroot and tip ends. The lightning protection system may comprise anoutboard down conductor disposed in the outboard portion of the blade.The outboard down conductor may comprise insulation which has anelectrical breakdown voltage higher than the insulation of the cablesecond portion, and optionally a higher electrical breakdown voltagethan the insulation of the cable first portion.

The outboard down conductor may comprise insulation which has a higherelectrical breakdown voltage than the cable second portion insulationelectrical breakdown voltage.

The wind turbine blade may further comprise an electrically conductivemetal foil on an outer surface of the blade, wherein the down conductorcable is either:

-   arranged electrically in parallel with the metal foil; or-   arranged electrically in series with the metal foil.

The, or at least one of the, down conductor cables may be spaced apartfrom at least one electrically conductive component of the blade.

The cable first portion may overlap at least partly with the at leastone electrically conductive component of the blade. This overlap may bein a spanwise direction of the blade.

The at least one electrically conductive component may comprise astructural component. The at least one electrically conductive componentmay comprise a spar cap. The spar cap may comprise carbon fibres. Thecarbon fibres may be pultruded carbon fibres.

The cable second portion of the second aspect of the invention may be atleast partly comprised in the plurality of inboard down conductor cablesof the first aspect of the invention. The plurality of inboard downconductor cables may each have insulation having a different electricalbreakdown voltage to one another. At least one of the down conductorcables may have insulation having the same electrical breakdown voltageas the cable first portion insulation.

The inboard portion may extend up to 40% of the spanwise dimension fromthe root end towards the tip end; preferably up to 30% of the spanwisedimension from the root end towards the tip end.

The or each cable may comprise a sheath. The or each cable’s respectiveinsulation having an electrical breakdown voltage may be provided by thesheath.

A second aspect of the invention provides a wind turbine bladecomprising:

-   a root end and a tip end, a spanwise dimension extending between the    root and tip ends; and an inboard portion extending up to 50% of the    spanwise dimension from the root end towards a midpoint between the    root and tip ends;-   at least one electrically conductive component disposed at least    partly in the inboard portion; and-   a lightning protection system comprising:    -   a down conductor;    -   a diverging electrical junction disposed in the inboard portion;        and    -   a plurality of inboard down conductor cables arranged        electrically in parallel to one another, disposed in the inboard        portion;    -   wherein the lightning protection system is configured so as to        conduct electricity through the down conductor, through the        diverging electrical junction, through the plurality of inboard        down conductor cables, towards the root end of the blade.

The second aspect of the invention has the advantage of reducing thepotential difference between the lightning protection system and anyother electrically conductive component of the blade. The plurality ofinboard down conductor cables are disposed between the divergingelectrical junction and the root end. Specifically, the plurality ofinboard down conductor cables being arranged electrically in parallel toone another reduces the current passing through each individual downconductor cable, and the potential difference between each individualdown conductor cable and any other electrically conductive component ofthe blade is reduced. This is particularly advantageous in the inboardportion of the blade as the potential difference increases along thelightning protection system from the tip end towards the root end of theblade during a lightning strike.

The plurality of inboard down conductor cables may be spaced apart fromone another. The plurality of inboard down conductor cables may bespaced apart from one another away from the diverging electricaljunction.

The plurality of inboard down conductor cables may comprise at least onedown conductor cable on or proximate the blade neutral axis.

The plurality of inboard down conductor cables may comprise at least oneinboard down conductor cable substantially disposed between a leadingedge and a shear web of the blade. Alternatively or in addition, theplurality of inboard down conductor cables may comprise at least oneinboard down conductor cable substantially disposed between a trailingedge and a shear web of the blade. Alternatively or in addition, theplurality of inboard down conductor cables may comprise at least oneinboard down conductor cable substantially disposed at or proximate tothe leading edge of the blade.

The plurality of inboard down conductor cables may comprise at least oneinboard down conductor cable attached to an interior surface of an outershell of the blade.

At least one of the inboard down conductor cables may be arranged in anat least partly meandering configuration. Alternatively or in addition,at least one of the inboard down conductor cables may be arranged so asto be substantially straight.

The plurality of inboard down conductor cables may comprise at least oneinboard down conductor cable substantially disposed at or proximate ashear web of the blade.

At least one of the plurality of inboard down conductor cables may passthrough an aperture in a shear web of the blade.

The plurality of inboard down conductor cables may comprise at leastthree down conductor cables.

The inboard portion may extend up to 40% of the spanwise dimension fromthe root end towards the tip end; preferably up to 30% of the spanwisedimension from the root end towards the tip end.

At least part of the down conductor may comprise an electricallyconductive metal foil on an outer surface of the blade. The metal foilmay comprise aluminium. At least one of the plurality of inboard downconductor cables may be arranged electrically in series with the metalfoil.

The metal foil may form the diverging electrical junction and theplurality of inboard down conductor cables extend from the metal foiltowards the root end.

The down conductor may comprise a down conductor cable and the divergingelectrical junction may split the down conductor cable into theplurality of inboard down conductor cables.

The lightning protection system may comprise a converging electricaljunction disposed in the inboard portion. The converging electricaljunction may be electrically connected to the plurality of inboard downconductor cables. The converging electrical junction may be disposedcloser to the root end of the blade than the plurality of inboard downconductor cables.

The converging electrical junction may be disposed closer to a root endof the blade than the electrically conductive component. The convergingelectrical junction may be spaced apart from the electrically conductivecomponent.

The diverging electrical junction may be arranged such that theplurality of inboard down conductor cables at least partly overlap withthe electrically conductive component.

The spanwise dimension may have an outboard portion extending up to 50%of the spanwise dimension from the tip end to a midpoint between theroot and tip ends. The lightning protection system may comprise anoutboard down conductor disposed in the outboard portion of the blade.

The outboard down conductor may be an outboard down conductor cable.

The at least one electrically conductive component may be a structuralcomponent. The at least one electrically conductive component maycomprise a spar cap. The spar cap may comprise carbon fibres. The carbonfibres may be pultruded carbon fibres.

A third aspect of the invention provides a wind turbine bladecomprising:

-   a root end and a tip end, a spanwise dimension extending between the    root and tip ends; and an inboard portion extending up to 50% of the    spanwise dimension from the root end towards a midpoint between the    root and tip ends;-   at least one electrically conductive component disposed in the    inboard portion; and-   a lightning protection system comprising:    -   a down conductor cable portion, disposed inside the inboard        portion of the blade; and    -   a supporting component,-   wherein the down conductor cable portion is secured in free space by    the supporting component in a position near or on the camber line of    the blade aerofoil section, spaced away from the at least one    electrically conductive component.

The third aspect of the invention has the advantage of permitting thedistance between the lightning protection system and any otherelectrically conductive component of the blade to be maximised, reducingthe likelihood of interaction between the lightning protection systemand any other electrically conductive component of the blade.

The down conductor cable portion may be located as far away as possiblefrom any or each electrically conductive component of the blade.

The at least one electrically conductive component may comprise astructural component. The at least one electrically conductive componentmay comprise a spar cap. The spar cap may comprise carbon fibres. Thecarbon fibres may be pultruded carbon fibres.

The blade may comprise two shell halves. Each shell half may have anelectrically conductive component. The down conductor cable portion maybe spaced substantially equally from both electrically conductivecomponents.

The blade may comprise at least one shear web. The down conductor cableportion may be secured in free space by the supporting component, spacedaway from the at least one shear web.

The supporting component may be attached to the at least one shear web.

The supporting component may be attached to the at least one shear webat at least two locations on the shear web. The blade may comprise atleast two shear webs. The supporting component may be attached to morethan one of the at least two shear webs. The down conductor cableportion may be disposed substantially centrally between a pair of the atleast two shear webs.

The supporting component may be one or more of: a foam piece; a foamsack; a bracket, optionally an X-shaped bracket, a C-shaped bracket, aY-shaped bracket; or a straight beam.

The down conductor cable portion may at least partly overlap with theelectrically conductive component in a spanwise direction.

Features of the first, second and third aspects of the invention may becombined with each other. In particular, the down conductor cableportion of the third aspect of the invention may be at least a portionof any of the plurality of inboard down conductor cables of the secondaspect of the invention.

The wind turbine blade of the first aspect of the invention may have thefeatures of the third aspect of the invention. The down conductor cableportion of the third aspect of the invention may be at least one of theplurality of inboard down conductor cables of the second aspect of theinvention.

At least one of the inboard down conductor cables may be spaced apartfrom an electrically conductive portion of one or more of: a sensorsystem; a de-icing system; an anti-icing system, a lighting system; aload control system, or any other wired system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows a wind turbine;

FIG. 2 shows a wind turbine blade;

FIG. 3 shows a layout of a blade having a known lightning protectionsystem;

FIG. 4 shows part of an inboard portion of a blade having a lightningprotection system of a first example;

FIG. 5 shows a perspective view of the portion of the blade of FIG. 4 ;

FIG. 6 shows a part of an inboard portion of a blade having a lightningprotection system of the first example;

FIG. 7 shows a cross-section through a blade having a lightningprotection system of the first example;

FIG. 8 shows a layout of a blade having a lightning protection system ofa second example;

FIG. 9 shows a cross-section of a blade having a lightning protectionsystem of a third example;

FIGS. 10 to 15 show a cross-section of various embodiments of a bladehaving a lightning protection system of the third example;

FIG. 16 shows a perspective view of a portion of a blade having thefirst, second and third examples.

DETAILED DESCRIPTION OF EMBODIMENT(S)

In this specification, terms such as root end, tip end, inboard,outboard, spanwise, leading edge, trailing edge, spar cap, shear web,outer shell, and camber line, are used. While these terms are well knownand understood to a person skilled in the art, definitions are givenbelow for the avoidance of doubt.

The term “root” used herein in relation to a blade end, refers to an endof the blade at which the blade is attached to a hub 20 of a turbine 1.The term “tip” used herein in relation to a blade end, refers to a bladeend that is radially furthest away from the axis of rotation 2 of thehub 20.

The term “spanwise” is used to refer to refer to a dimension ordirection from a root end of a wind turbine blade to a tip end of theblade, or vice versa. When a wind turbine blade is mounted on a windturbine hub, the spanwise and radial directions of the wind turbineblade will be substantially the same.

The term “inboard” is used to refer to a portion of the blade closer tothe root end than the tip end in the spanwise direction. The term“outboard” is used to refer to a portion of the blade closer to the tipend than the root end in the spanwise direction.

The term “leading edge” is used to refer to an edge of the blade whichwill be at the front of the blade as the blade rotates in the normalrotation direction of the wind turbine rotor.

The term “trailing edge” is used to refer to an edge of a wind turbineblade which will be at the back of the blade as the blade rotates in thenormal rotation direction of the wind turbine rotor.

The chord of a blade is the straight line distance from the leading edgeto the trailing edge in a given cross section perpendicular to the bladespanwise direction.

A pressure surface (or windward surface) of a wind turbine blade is asurface between the leading edge and the trailing edge, which, when inuse, has a higher pressure than a suction surface of the blade.

A suction surface (or leeward surface) of a wind turbine blade is asurface between the leading edge and the trailing edge, which will havea lower pressure acting upon it than that of a pressure surface, when inuse.

The thickness of a wind turbine blade is measured perpendicularly to thechord of the blade and is the greatest distance between the pressuresurface and the suction surface in a given cross section perpendicularto the blade spanwise direction.

The term “spar cap” is used to refer to a longitudinal, generallyspanwise extending, reinforcing member of the blade. The spar cap may beembedded in the blade shell, or may be attached to the blade shell. Thespar caps of the windward and leeward sides of the blade may be joinedby one or more shear webs extending through the interior hollow space ofthe blade. The blade may have more than one spar cap on each of thewindward and leeward sides of the blade. The spar cap may form part of alongitudinal reinforcing spar or support member of the blade. Inparticular, a first and a second spar cap may form part of the loadbearing structure extending in the longitudinal direction that carriesthe flap-wise bending loads of the blade.

The term “shear web” is used to refer to refer to a longitudinal,generally spanwise extending, reinforcing member of the blade that cantransfer load from one of the windward and leeward sides of the blade tothe other of the windward and leeward sides of the blade.

The term “shell” is used to refer to an outer aerodynamic structure ofthe blade. The shell may be provided as two shell halves, which may bebonded together by an adhesive.

The term “camber line” is used to refer to a line upon which a profilethickness distribution is symmetrically superimposed. In a typical windturbine blade the camber line will be curved.

The term “breakdown voltage” is the voltage which causes a severe lossof the insulating properties of the cable’s insulation

FIG. 1 shows a wind turbine 10 including a tower 12 and a nacelle 14disposed at the apex of the tower 12.

A rotor 16 is operatively coupled via a gearbox to a generator (notshown) housed inside the nacelle 14. The rotor 16 includes a central hub18 and a plurality of rotor blades 20, which project outwardly from thecentral hub 18. While the embodiment shown in FIG. 1 has 3 blades, itwill be understood by the skilled person that other numbers of bladesare possible.

When wind blows against the wind turbine 10, the blades 20 generate alift force which causes the rotor 16 to rotate, which in turn causes thegenerator within the nacelle 14 to generate electrical energy.

FIG. 2 illustrates a wind turbine blade 20 for use in such a windturbine 10. The wind turbine blade 20 has a root end 21 and a tip end22. The wind turbine blade 20 has a leading edge 28 and a trailing edge29.

With reference to FIG. 3 , the wind turbine blade 20 has a prior artlightning protection system 11. The lightning protection system 11provides a lightning current path from a variety of likely lightningattachment points on the blade, via a down conductor system, to the rootend 21 of the blade. At the root end 21 there may be electricallyconductive slip rings or bands which provide electrical connection tothe hub 18 and/or nacelle 14 of the wind turbine 10 yet allow pitchrotation of the blade relative to the hub. The hub 18 and/or nacelle 14are electrically connected to ground to provide a lightning current pathfrom the blade to ground.

The down conductor of the lightning protection system 11 may comprise: atip down conductor cable 15; an electrically conductive metal foil 17just inboard of an outer surface of the blade 20; and an inboard downconductor cable 41. There may be provided a layer, e.g. a glass layer,covering the electrically conductive metal foil 17. The electricallyconductive metal foil 17 may have a mesh configuration or be an expandedmetal foil. The blade 20 may have a lightning tip receptor such as asolid metal tip and a plurality of discrete lightning receptors on thesurface of the blade 20 near the tip 22. The solid metal tip and thediscrete lightning receptors may be electrically connected to the tipdown conductor cable 15. The root end of the tip down conductor cable 15may be electrically connected to the tip end of the electricallyconductive metal foil 17. The tip end of the inboard down conductorcable 41 may be electrically connected to the root end of theelectrically conductive metal foil 17. The root end of the inboard downconductor cable 41 may be electrically connected to the slip rings atthe root end 21 of the blade. The electrically conductive metal foil 17may be omitted and the down conductor may be a cable extending from theroot end to the tip end of the blade. The down conductor mayalternatively have a down conductor cable extending in parallel with theconductive metal foil over part of the blade length.

The wind turbine blade 20 has a spanwise dimension 25 extending betweenthe root and tip ends 21, 22; and an inboard portion 23 extending up to50% of the spanwise dimension 25 from the root end 21 towards a midpoint26 between the root and tip ends 21, 22, for example, as shown in FIG. 2. The inboard portion 23 may extend up to 40% of the spanwise dimensionfrom the root end 21 towards the tip end 22; preferably up to 30% of thespanwise dimension 25 from the root end 21 towards the tip end 22. Theblade 20 may also have an outboard portion 24 extending at least up tothe inboard portion 23, of the spanwise dimension 25 from the tip end 22to the root end 21. The outboard portion 24 may extend up to 50% of thespanwise dimension 25 from the tip end 22 towards the root end 21.

The wind turbine blade 20 may also comprise at least one electricallyconductive component 31, 32. The at least one electrically conductivecomponent 31, 32 is disposed at least partly in the inboard portion 23.The at least one electrically conductive component 31, 32 may comprise astructural spar cap. The spar cap 31, 32 may comprise carbon fibres. Thecarbon fibres may be pultruded carbon fibres. The pultruded carbonfibres may extend substantially spanwise along a portion of the blade20. There may be provided a first and a second spar cap. The first sparcap may be provided on a pressure side of the blade. The second spar capmay be provided on a suction side of the blade. There may be one or moreshear webs extending between the first and second spar caps. There maybe provided a leading pair and a trailing pair of spar caps. Each pairof spar caps may comprise a first and a second spar cap, and optionallya shear web extending therebetween. Each of the first and second sparcaps may be integrated into the shell of the blade 20. Alternatively,the spar caps may be attached to an interior surface of the shell of theblade 20. Further alternatively some of the spar caps may be integratedinto the shell and some of the spar caps may be attached to an interiorsurface of the shell.

FIGS. 4 to 7 show a first example of a lightning protection system 100with a diverging electrical junction to distribute lightning current inorder to minimise any interaction between the lightning protectionsystem and an electrically conductive component 31, 32.

The lightning protection system 100 comprises: a down conductor 110; adiverging electrical junction 120 and a plurality of inboard downconductor cables 131, 132, 133. The lightning protection system 100 isconfigured so as to conduct electricity through the down conductor 110,through the diverging electrical junction 120, through the plurality ofinboard down conductor cables 131, 132, 133, towards the root end 21 ofthe blade 20. An advantage of the diverging junction 120 to split thedown conductor cable 110 into a plurality of inboard down conductorcables 131, 132, 133 is that the current passing through each of theplurality of inboard down conductor cables 131, 132, 133 is reduced.This reduces the potential difference between an electrically conductivecomponent and any individual down conductor cable 131, 132, 133, and inturn reduces the likelihood of flashover between one of the downconductor cables 131, 132, 133 and an electrically conductive component.In addition, the distribution of current between the plurality ofinboard down conductor cables reduces the heat buildup in these cablesand may also allow the use of smaller diameter cables.

The down conductor 110 may comprise a down conductor cable. The downconductor 110 may be at least partly disposed in the inboard portion 23of the blade 20. The down conductor 110 may be substantially or whollydisposed in the inboard portion 23 of the blade 20.

The diverging electrical junction 120 may be disposed in the inboardportion 23 of the blade 20. The diverging electrical junction 120 hasthe function of splitting out an electrical path from the down conductor110 into multiple paths, as the path moves from the tip end 22 to theroot end 21 within the inboard portion 23. The diverging electricaljunction 120 may be provided as a port or ports, to which the pluralityof inboard down conductor cables 131, 132, 133 may be connected. Theplurality of inboard down connector cables 131, 132, 133 may each have atip end, and may each be electrically connected to the divergingelectrical junction 120 at their tip end. The plurality of inboard downconnector cables 131, 132, 133 may each be connected to the electricaljunction 120 proximate one another. The plurality of inboard downconnector cables 131, 132, 133 may each be connected to the electricaljunction 120 so as to define acute angles therebetween, such that theplurality of inboard down connector cables 131, 132, 133 may extend awayfrom the diverging electrical junction 120 and in doing so extend awayfrom one another. The diverging electrical junction 120 may be arrangedsuch that the plurality of inboard down conductor cables 131, 132, 133at least partly overlap with the electrically conductive component 31,32. This overlap may be in a spanwise direction of the blade 20.

The plurality of inboard down conductor cables 131, 132, 133 may bearranged electrically in parallel to one another, and may be disposed inthe inboard portion 23. The plurality of inboard down conductor cables131, 132, 133 may each be provided with an electrically conductive core,and/or an electrically insulative sheath. The plurality of inboard downconductor cables 131, 132, 133 may each be substantially flexible. Theplurality of inboard down conductor cables 131, 132, 133 may each be thesame or substantially the same as one another. The plurality of inboarddown conductor cables 131, 132, 133 may comprise any appropriate numberof down conductor cables. The plurality of inboard down conductor cables131, 132, 133 may comprise at least three down conductor cables, forexample as shown in FIGS. 4 to 7 . The plurality of inboard downconductor cables 131, 132, 133 may be arranged so as to extend insubstantially the same direction as a spanwise extension of a shear webof the blade. The plurality of inboard down conductor cables 131, 132,133 may be arranged so as to extend to each side of a shear web of theblade. The plurality of inboard down conductor cables 131, 132, 133 maybe arranged such that the shear web extends at least partly between atleast two of the plurality of inboard down conductor cables 131, 132,133.

The plurality of inboard down conductor cables 131, 132, 133 may bespaced apart from one another away from the diverging electricaljunction 120. The plurality of inboard down conductor cables 131, 132,133 may comprise at least one down conductor cable 131, 132, 133 on orproximate a blade neutral axis.

The plurality of inboard down conductor cables 131, 132, 133 maycomprise at least one down conductor cable 131, 132, 133 substantiallydisposed between a leading edge 28 and a shear web of the blade 20, orbetween a trailing edge 29 and a shear web of the blade 20, orsubstantially disposed at or proximate to the leading edge 28 of theblade 20.

The plurality of inboard down conductor cables 131, 132, 133 maycomprise at least one down conductor cable 131 attached to an interiorsurface of an outer shell of the blade 20, for example as shown in FIG.7 . At least one of the down conductor cables 131, 132, 133 may bearranged in an at least partly meandering configuration, or arranged soas to be substantially straight, for example as shown in FIGS. 5 and 6 ,in which the outer two of the plurality of inboard down conductor cables131, 133 are in an at least partly meandering configuration, and theinner cable 132 is substantially straight. The meandering configurationmay be configured so as to accommodate any dimensional changes of anycomponent to which the cable is attached. For example, away from theblade neutral axis, the blade structure will bend under load and it isundesirable to transfer this bending load to the down conductor cable.By providing a meandering path for the down conductor cable, the cablecan be insulated from these blade bending loads. Conversely, at theneutral axis of the blade, the down conductor cable path may besubstantially straight, so as to save the cost and weight of unnecessarycable length.

The plurality of inboard down conductor cables 131, 132, 133 maycomprise at least one down conductor cable substantially disposed at orproximate a shear web of the blade 20, for example as shown with theinner cable 132 in FIG. 6 . At least one of the plurality of inboarddown conductor cables 131, 132, 133 may be attached to a shear web ofthe blade 20. This attachment may be such that the at least one of theplurality of inboard down conductor cables 131, 132, 133 is restrictedor prevented from movement in a chordwise direction of the blade 20.

At least one of the down conductor cables 131, 132, 133 may pass throughan aperture in a shear web of the blade 20. The aperture may beconfigured having a diameter through which the at least one of the downconductor cables can pass. An area of the aperture may correspond to across-sectional area of the cable that passes therethrough, e.g. so asto provide a clearance fit between the cable and the shear web. Theshear web may have a height (in the blade thickness direction), and theaperture may be provided mid-way along the height of the shear web.

The lightning protection system 100 may comprise an outboard downconductor 110 disposed in the outboard portion 24 of the blade 20. Theoutboard down conductor 110 may be the same or substantially the same asany of the down conductors described in relation to FIG. 3 , or shown inFIG. 3 .

The wind turbine blade 20 may further comprise an electricallyconductive metal outer foil 17 on an outer surface of the blade 20. Themetal foil 17 may extend around an outside of the blade, so as to coveran outer surface of the shell of the blade 20. The metal foil may extendalong only a part of the span of the blade 20. The metal foil 17 may bearranged so as to be in both of the inboard and outboard portions 23, 24of the blade 20. The metal foil 17 may be disposed mostly in an outboardportion 24 of the blade 20. The metal foil 17 may be a curved sheet ofmetal. The metal foil 17 may be substantially frustoconical in shape.

At least one of the plurality of inboard down conductor cables 131, 132,133 may be either:

-   arranged electrically in parallel with the metal foil 17; or-   arranged electrically in series with the metal foil 17.

Where at least one of the plurality of inboard down conductor cables131, 132, 133 is arranged electrically in series with the metal foil 17,the down conductor cable 131, 132, 133 may be attached to the metal foil17. The attachment may be achieved directly and/or may be achieved by anattachment component.

The outer foil 17 may be the diverging electrical junction 120. Forexample, at least two of the plurality of inboard down conductor cables131, 132, 133 may be attached to the outer foil 17, for example at aroot end of the outer foil 17.

The lightning protection system 100 may comprise a converging electricaljunction 140 disposed in the inboard portion 23, electrically connectedto the plurality of inboard down conductor cables 131, 132, 133. Theconverging electrical junction 140 may be disposed closer to the rootend 21 of the blade 20 than the plurality of inboard down conductorcables 131, 132, 133. The converging electrical junction 140 has thefunction of joining electrical paths from the plurality of inboard downconductor cables 131, 132, 133, into less paths, preferably a singlepath, as the paths move towards the root end 21, within the inboardportion 23.

The converging electrical junction 140 may be disposed closer to theroot end 21 of the blade 20 than the electrically conductive component31, 32. At a location closer to the root end 21 of the blade 20 than theelectrically conductive component 31, 32, there is a significantlyreduced risk of flashover between a down conductor cable and theelectrically conductive component. For example, the electricityconductive component may be terminated at a spanwise distance furtheroutboard than the converging electrical junction 140. Inboard of theconverging electrical junction 140, the plurality of conductor cables131, 132, 133 can be brought together to form a single or reduced numberof current flow paths in a single or reduced number of down conductorcomponents or cables.

The converging electrical junction 140 may be spaced apart from theelectrically conductive component 31, 32. The converging electricaljunction 140 may be secured in place, so as to be held in place awayfrom the electrically conductive component 31, 32. The convergingelectrical junction 140 may be substantially the same in shape, size,materials, and/or configuration as the diverging electrical junction120. The converging electrical junction 140 may be arranged so as tosubstantially be a mirror-reflection of the diverging electricaljunction 120. The converging electrical junction 140 and attached downconductor cables 131, 132, 133 may be arranged so as to substantially bea mirror-reflection of the diverging electrical junction 120 andattached down conductor cables 131, 132, 133.

FIG. 8 shows a second example with a blade having a lightning protectionsystem where parts of the cable have different electrical insulationlevels, i.e. different electrical breakdown voltages. This allows theparts of the cables to be routed closer to electrically conductivecomponents 31, 32 and/or the prevention of flashover between thelightning protection system and the electrically conductive component.

The lightning protection system in FIG. 8 comprises a down conductorcable 230 at least partly disposed inside the inboard portion 23 of theblade 20. The down conductor cable 230 may be disposed substantially, orwholly, inside the inboard portion 23 of the blade 20. The downconductor cable 230 may extend to the root end 21 of the blade 20, or beelectrically connected to an electrically conductive component, such asa cable, that extends to the root end 21 of the blade 20.

The down conductor cable 230 comprises a cable first portion 231 and acable second portion 232. The cable first portion 231 is closer to theroot end 21 of the blade 20 than the cable second portion 232. Both ofthe first and second cable portions 231, 232 may be disposed in theinboard portion 23 of the blade 20. Each cable portion 231, 232 hasinsulation having an electrical breakdown voltage. The cable electricalbreakdown voltage of the first portion 231 insulation is higher than theelectrical breakdown voltage of the cable second portion 232 insulation.Approaching the root end 21 of the blade 22, the potential differencebetween any down conductor cable and any electrically conductivecomponent 31, 32 tends to increase. In turn, the likelihood of flashoverfrom a down conductor cable to an electrically conductive 31, 32component increases. By providing a first portion having insulationhaving a higher electrical breakdown voltage at a root end 21 of theblade 20, the risk of flashover is substantially mitigated at a locationwhere the potential difference is highest. By providing a first portion231 having a higher electrical breakdown voltage than a second portion,the flashover mitigation is heightened where the potential difference ishighest.

The cable second portion 232 may overlap at least partly with the atleast one electrically conductive component 31, 32 of the blade in aspanwise direction of the blade 20. The cable first portion 231 mayoverlap at least partly with the at least one electrically conductivecomponent 31, 32 of the blade in a spanwise direction of the blade 20.

The difference in electrical breakdown voltage between the cable firstportion 231 and the cable second portion 232 may arise from one or moreof: thickness; material; or shape of the insulation surrounding theconductor.

There may be provided a cable third portion 236, disposed closer to theroot end 21 of the blade 20 than the cable first portion 231, andelectrically connected to the cable first portion 231. The cable thirdportion 236 may have insulation having an electrical breakdown voltagelower than the electrical breakdown voltage of the cable first portion231 insulation. The cable third portion insulation electrical breakdownvoltage may be the same as the cable second portion 232 insulationelectrical breakdown voltage. The cable third portion 236 may bedisposed closer to the root end 21 of the blade 20 than one or moreelectrically conductive components 31, 32 of the blade 20. The cablethird portion 236 may be spaced away, in a spanwise direction, from oneor more electrically conductive components 31, 32 of the blade 20.

The lightning protection system 200 may comprise an outboard downconductor 210 disposed in the outboard portion 24 of the blade 20, theoutboard down conductor 210 having insulation having a higher breakdownvoltage than the cable second portion 232. The outboard down conductor210 may be an outboard down conductor cable. The outboard down conductor210 may have insulation having a breakdown voltage higher than the cablefirst portion 231 insulation. This has the advantage of preventingattachment of lightning directly to the outboard down conductor 210.This provides a more reliable path for current to follow which does notcircumvent the conductive solid metal tip 22 or other lightningreceptors of the blade 20. Instead the lightning will attach to the tip22 or other lightning receptors of the blade 20 and the current willthen pass through the outboard down conductor 210.

The, or at least one of the, down conductor cables 210, 230 may bespaced apart from at least one electrically conductive component 31, 32of the blade 20.

The wind turbine blade 20 may further comprise an electricallyconductive metal foil 17 as described previously. The down conductorcable 210 may arranged in relation to the metal foil 17 as describedpreviously.

The at least one electrically conductive component 31, 32 may comprise aspar cap, which may be configured as described previously.

The or each cable 210, 230 may comprise an electrically conductive coreand/or an electrically insulative sheath. The respective cableinsulation electrical breakdown voltage may be provided by the sheath.The sheath may be a layer, coating and/or be tubular in shape.

The down conductor cable 230 may be arranged in an at least partlymeandering configuration, for example as shown in FIG. 8 .

FIGS. 9 to 15 show a third example where the distance between a downconductor cable and any electrically conductive component can bemaximised.

The wind turbine blade 20 comprises at least one electrically conductivecomponent 31, 32 and a lightning protection system 300. The at least oneelectrically conductive component 31, 32 is disposed at least partly inthe inboard portion 23.

The lightning protection system 300 comprises a down conductor cableportion 330 and a supporting component 340.

The down conductor cable portion 330 is disposed inside the inboardportion 23 of the blade 20. The down conductor cable portion 330 issecured and suspended in free space by the supporting component 340 in aposition near or on the camber line of the blade aerofoil section, forexample as shown in FIG. 9 , spaced away from the at least oneelectrically conductive component 31, 32. As demonstrated by the arrowsin FIG. 9 , the down conductor cable portion 330 may be secured andsuspended in free space at a maximum distance from each of theelectrically conductive components 31, 32.

As a skilled person will appreciate, the term “free space” used hereinrefers to space within the blade 20, occupied by the down conductorcable portion 330 and the supporting component 340, that is not occupiedby another component of the blade 20. In particular, free space is notoccupied by any structural component of the blade 20. For example, spacethat accommodates a shear web is not free space. Space between two shearwebs, or a shear web and an edge of the blade, may be free space.

An advantage of utilising free space within the blade 20 to accommodatethe down conductor cable portion 330 is that the distance between thedown conductor cable portion 330 and any electrically conductivecomponent 31, 32 can be maximised. By increasing the distance betweenthe down conductor cable portion 330 and any electrically conductivecomponent, the risk of flashover between the down conductor cableportion 330 and any electrically conductive component may be eliminated.

The down conductor cable portion 330 may at least partly overlap with atleast one electrically conductive component 31, 32 in a spanwisedirection. The at least one electrically conductive component 31, 32 maycomprise a spar cap, which may be configured as described previously.

The blade 20 may comprise a shell structure as described previously. Inthe case of two shell halves, each shell half may have an electricallyconductive component. The down conductor cable portion 330 may be spacedsubstantially equally from both electrically conductive components 31,32. The shell halves may be manufactured separately and then joinedtogether during manufacture or the two shell halves may be manufacturedtogether as a single shell.

The blade 20 may comprise at least one shear web. The down conductorcable portion 340 may be secured in free space by the supportingcomponent 340, spaced away from the at least one shear web. Thesupporting component 340 may be attached to the at least one shear webor at least one shell. The supporting component 340 may be attached tothe at least one shear web at at least two locations on the shear web,for example as shown in FIGS. 10, 14 and 15 . The blade 20 may compriseat least two shear webs. The supporting component 340 may be attached tomore than one of the at least two shear webs, for example as shown inFIG. 10 . The down conductor cable portion 330 may be disposedsubstantially centrally between a pair of the at least two shear webs,for example as shown in FIGS. 9 and 10 .

The supporting component 340 may be one or more of: a foam piece; a foamsack; a bracket, optionally an X-shaped bracket, a C-shaped bracket, aY-shaped bracket; a straight beam, or rope or the like. The supportingcomponent may be electrically nonconductive so that it does notcompromise the down conductor system. The support component may also bedesigned to damp vibrations in the cable such that loads on the cableare reduced.

As shown in FIG. 10 , the down conductor cable portion 330 may be heldin place by a supporting component 340 having a plurality of struts 341,342, 343, 344. The plurality of struts may comprise a first strut 341attached to a first shear web, and/or a second strut 343 attached to asecond shear web. The supporting component 340 may comprise a pluralityof struts attached to a plurality of shear webs, such as two struts 341,342 attached to a first shear web, and/or two struts 343, 344 attachedto a second shear web. The struts 341, 342 attached to the first shearweb may be attached at a pressure side end and a suction side end of thefirst shear web respectively. Equally, the struts 343, 344 attached tothe second shear web may be attached pressure side end and a suctionside end of the second shear web respectively. The struts 341, 342, 343,344 may be arranged so as to extend towards a central point, at whichthe down conductor cable portion 330 is disposed. The struts 341, 342,343, 344 may define a cross-shape in cross-section, and may be, forexample, an X-shape bracket. The struts 341, 342, 343, 344 may befixedly attached to a central supporting component 345. The centralsupporting component 345 may be fixedly attached to at least part of thedown conductor portion 330, and/or act as a housing to at least part ofthe down conductor portion 330. The central supporting component 345 maydefine a ring-shape, and/or be substantially cylindrical.

As shown in FIG. 11 , the down conductor cable portion 330 may be heldin place by a supporting component 340 having a plurality of pieces 346,347. The plurality of pieces 346, 347 may secure the down conductorcable portion 330 between a leading edge 28 and a trailing edge 29 ofthe blade 20. The plurality of pieces 346, 347 may secure the downconductor cable portion 330 in compression between the plurality ofpieces 346, 347. Additional attachment means may be provided, such as anattachment component, bracket, or adhesive. Additional attachment meansmay secure the down conductor cable portion 330 to at least one of theplurality of pieces 346, 367, and/or secure at least one of theplurality of pieces 346, 347 to a suction side 51 or a pressure side 52of the blade 20. The plurality of pieces 346, 347 may be substantiallysolid, and/or wedge-shaped. The plurality of pieces 346, 347 may fill atleast half of the space between at least two shear webs in the blade 20.The plurality of pieces 346, 367 may substantially fill the spacebetween at least two shear webs. The plurality of pieces 346, 347 may beelastically compressible. The plurality of pieces 346, 347 may compriseor consist of foam. The or each foam piece 346 may be arranged so as tocontact either a suction side 51 or a pressure side 52 of the blade 20.One of the plurality of pieces 346 may be arranged so as to contact,and/or be attached to, the suction side 51 of the blade 20. The other ofthe plurality of pieces 347 may be arranged so as to contact, and/or beattached to, the pressure side 52 of the blade 20. The plurality ofpieces 346, 347 may be arranged so as to not bear against or be incontact with any shear web of the blade 20. The foam pieces 346, 347 maydefine an aperture therebetween, in which the down conductor cableportion 330 may be disposed or secured.

As shown in FIGS. 12 and 13 , the down conductor cable portion 330 maybe held in place by a supporting component 340 that is provided as asack 348. The sack 348 may be inflatable. A partially inflated view ofan embodiment of a sack 348 is shown in FIG. 12 , and a fully inflatedview of the sack 348 of FIG. 12 is shown in FIG. 13 . The sack 348 maybe provided with an aperture, which may be a central aperture, in whichthe down conductor cable portion 330 may be disposed. The sack 348 mayat least substantially fill a free space in the blade 20, for example asshown in FIG. 13 . The sack 348 may, in an inflated state, bear againsta suction side 51 and/or a pressure side 52 of the blade 20. The sack348 may, in an inflated state, bear against at least one shear web. Thesack 348 may be disposed between two shear webs, and may bear againstboth shear webs in an inflated state. The sack 348 may bear against atleast part of all of: the suction side 51, the pressure side 52, a firstshear web, and a second shear web of the blade 20. The sack 348 may beelastically compressible in its inflated state. The sack 348 may befilled with a cured or curable material, such as a polymeric material.The sack 348 may be a foam sack.

As shown in FIG. 14 , the down conductor cable portion 330 may be heldin place by a bracket 349. The bracket 349 may be attached to a shearweb. There may be provided a leading and a trailing shear web. Thebracket 349 may be attached to the trailing shear web. The bracket 349may at least partly extend towards a trailing edge 29 of the blade 20.The bracket 349 may be elastically compressible. The bracket 349 may besubstantially C-shaped. The bracket 349 may be attached to a centralsupporting component 345. The central supporting component 345 may befixedly attached to at least part of the down conductor portion 330,and/or act as a housing to at least part of the down conductor portion330. The central supporting component 345 may define a ring-shape,and/or be substantially cylindrical.

As shown in FIG. 15 , the down conductor cable portion 330 may be heldin place by a bracket 350. The bracket 350 may be attached to a shearweb. There may be provided a leading and a trailing shear web. Thebracket 350 may be attached to the leading shear web. The bracket 350may at least partly extend towards a leading edge 28 of the blade 20.The bracket 350 may be elastically compressible. The bracket 350 may besubstantially Y-shaped. The bracket 350 may be attached to a centralsupporting component 345. The central supporting component 345 may befixedly attached to at least part of the down conductor portion 330,and/or act as a housing to at least part of the down conductor portion330. The central supporting component 345 may define a ring-shape,and/or be substantially cylindrical. The bracket 350 may comprise aplurality of struts 351, 352, 353. The plurality of struts 351, 352, 353may be attached to and/or extend from the central supporting component350. The central supporting component may be disposed between a leadingedge 28 of the blade and a shear web, which may be a leading shear web.A first strut 351 of the plurality of struts 351, 352, 353 may beattached to and/or extend from a suction side end 51 of a shear web,which may be the leading shear web. A second strut 352 of the pluralityof struts 351, 352, 353 may be attached to and/or extend from a pressureside end 52 of a shear web, which may be the leading shear web. A third353 of the plurality of struts 351, 352, 353 may be attached to and/orextend from a leading edge 28 of the blade 20.

As a skilled person will appreciate, various features of each of theabove examples, and any or all of the described examples, may becombined. FIG. 16 shows an example having features of each of the threedescribed examples above.

In FIG. 16 , the down conductor 133 may be secured in free space withinthe blade shell by at least one floating conductor suspension strut.There may be provided at least one supporting component 240 in the formof at least one or a plurality of floating conductor suspension struts,which may support the down conductor 133. The down conductor 133 may beattached to the suspension struts, and/or pass through at least oneaperture in a suspension strut. Each suspension strut may be providedwith an aperture, dimensioned for the down conductor 133 to passtherethrough. The or each aperture may be provided centrally on therespective suspension strut. Each aperture may be aligned with at leastone other aperture, or they may all be substantially aligned, so thatthe down conductor can be held in a substantially straight position. Theor each suspension strut may be configured to bridge a plurality, forexample two, shear webs. The or each suspension strut may be attached toat least two shear webs. The or each suspension strut may besubstantially elongate, rectangular, and/or planar.

With reference to FIG. 16 , each of the plurality of inboard downconductor cables 131, 132, 133 may each have insulation having differentelectrical breakdown voltages to one another. For example, the downconductor cable 131 provided at a leading edge 28 of the blade 20 mayhave a lower electrical breakdown voltage than at least one or each ofthe down conductor cables 132, 132 provided closer to the shear webs,and in turn closer to the electrically conductive components 31, 32 ofthe blade 20. At an outboard end of the blade portion shown in FIG. 16 ,there may be provided a down conductor 110, having a lower electricalbreakdown voltage than a down conductor portion provided furtherinboard, thus benefiting from the advantages of the second example.Where one or more of the cables’ electrical breakdown voltage is lower,a cost reduction and/or a mass reduction may be achieved.

The wind turbine blade 20 of the first example may include any or allfeatures of the second example.

The cable second portion 232 of the second example may be at leastpartly comprised in one or more of the plurality of inboard downconductor cables 130 of the first example. The plurality of inboard downconductor cables 131, 132, 133 may have insulation having differentelectrical breakdown voltages to one another. At least one of the downconductor cables 131, 132, 133 may have insulation having the sameelectrical breakdown voltage as the cable first portion 231 insulationelectrical breakdown voltage.

The down conductor cable portion 330 of the third example may be atleast a portion of the down conductor cable 110, or any of the pluralityof inboard down conductor cables 131, 132, 133 of the first example. Thedown conductor cable portion 330 of the third example may be any otherdown conductor cable of the wind turbine blade 20.

The down conductor cable portion 330 of the third example may be atleast one of the plurality of inboard down conductor cables 131, 132,133 of the first example.

At least one of the down conductor cables of any of the first, second,and third examples may be spaced apart from an electrically conductiveportion of one or more of; a sensor system; a de-icing system; alighting system; a load control system.

In the present disclosure, the term ‘cable’ also includes conductivebands, straps and braids.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

Although a blade construction involving two spar caps and webs has beenshown in some of the figures, the skilled person will appreciate thatthe examples may be implemented in other blade structures.

Although an example having three down conductor cables is described withrespect to the first example, the plurality of inboard down conductorcables 131, 132, 133 may comprise only two down conductor cables, four,five, or any appropriate number.

Although an example having a converging electrical junction 140 to joinone or more of a plurality of inboard down conductor cables 131, 132,133 is described, as an alternative, at least two or all of theplurality of inboard down conductor cables may be directly or indirectlyjoined to a slip band at the root end 21 of the blade 20.

Although examples have been described where the electrically conductivecomponent 31, 32 comprises spar cap, the electrically conductivecomponent may be other conductive features in the blade, such as anysystem which has electrical wiring. The examples described above willprotect these conductive features from lightning current.

1. A wind turbine blade comprising: a root end and a tip end, a spanwisedimension extending between the root and tip ends; and an inboardportion extending up to 50% of the spanwise dimension from the root endtowards a midpoint between the root and tip ends; a lightning protectionsystem comprising a down conductor cable at least partly disposed insidethe inboard portion of the blade, wherein the down conductor cablecomprises a cable first portion and a cable second portion, the cablefirst portion being closer to the root end of the blade than the cablesecond portion, each cable portion comprises insulation which has arespective electrical breakdown voltage, wherein the cable first portioninsulation electrical breakdown voltage is higher than the cable secondportion insulation electrical breakdown voltage.
 2. The wind turbineblade of claim 1, wherein the cable first portion comprises insulationwhich has an electrical breakdown voltage at least 1.2 times higher thanthe electrical breakdown voltage of the cable second portion insulation.
 3. The wind turbine blade of claim 1, wherein the spanwise dimensionhas an outboard portion extending up to 50% of the spanwise dimensionfrom the tip end to a midpoint between the root and tip ends, whereinthe lightning protection system comprises an outboard down conductordisposed in the outboard portion of the blade, the outboard downconductor having insulation having a higher electrical breakdown voltagethan the cable second portion, and optionally a higher breakdown voltagethan the cable first portion.
 4. The wind turbine blade of claim 3,wherein the outboard down conductor comprises insulation which has anelectrical breakdown voltage that is higher than the cable secondportion insulation electrical breakdown voltage.
 5. The wind turbineblade of claim 1, wherein the, or at least one of the down conductorcables is spaced apart from at least one electrically conductivecomponent of the blade.
 6. The wind turbine blade of claim 5, whereinthe cable first portion overlaps at least partly with the at least oneelectrically conductive component of the blade in a spanwise directionof the blade.
 7. The wind turbine blade of claim 5, wherein the at leastone electrically conductive component is a structural componentcomprising a spar cap .
 8. The wind turbine blade of claim 1, furthercomprising at least one electrically conductive component disposed inthe inboard portion, and wherein the lightning protection system furthercomprises: a down conductor; a diverging electrical junction disposed inthe inboard portion; and a plurality of inboard down conductor cablesarranged electrically in parallel to one another, disposed in theinboard portion; wherein the lightning protection system is configuredso as to conduct electricity through the down conductor, through thediverging electrical junction, through the plurality of inboard downconductor cables, towards the root end of the blade.
 9. The wind turbineblade of claim 8, wherein the cable second portion is at least partlycomprised in the down conductor and/or at least one of the plurality ofinboard down conductor cables.
 10. The wind turbine blade of claim 8,wherein the plurality of inboard down conductor cables has insulationhaving different electrical breakdown voltages to one another.
 11. Thewind turbine blade of claim 10, wherein at least one of the plurality ofinboard down conductor cables has insulation having the same electricalbreakdown voltage as the cable second portion insulation electricalbreakdown voltage.
 12. The wind turbine blade of claim 1, furthercomprising at least one electrically conductive component disposed inthe inboard portion, and wherein the lightning protection system furthercomprises a supporting component, wherein the down conductor cablecomprises a portion secured in free space by the supporting component ina position near or on the camber line of the blade aerofoil section,spaced away from the at least one electrically conductive component. 13.The wind turbine blade of claim 1, wherein the inboard portion extendsup to 40% of the spanwise dimension from the root end towards the tipend .
 14. The wind turbine blade of claim 1, wherein at least one of thedown conductor cables is spaced apart from an electrically conductiveportion of one or more of; a sensor system; a de-icing system, ananti-icing system; a lighting system; and a load control system.
 15. Thewind turbine blade of claim 1, wherein the cable first portion comprisesinsulation which has an electrical breakdown voltage at least 1.5 timeshigher than the electrical breakdown voltage of the cable second portioninsulation.
 16. The wind turbine blade of claim 1, wherein the cablefirst portion comprises insulation which has an electrical breakdownvoltage at least 1.6 times higher than the electrical breakdown voltageof the cable second portion insulation.
 17. The wind turbine blade ofclaim 7, wherein the spar cap comprises carbon fibres.
 18. The windturbine blade of claim 17, wherein the carbon fibres are pultrudedcarbon fibres.
 19. The wind turbine blade of claim 1, wherein theinboard portion extends up to 30% of the spanwise dimension from theroot end towards the tip end.